1. Field of the Invention
The present invention relates to a method and an apparatus for performing a cell search in a wireless communication system.
2. Description of the Related Art
A typical wireless communication system includes at least one cell to provide a communication service to each subscriber station (mobile terminal). Each cell may be divided into a predetermined number of segments according to the number of subscribers. A subscriber station must recognize the cell to which the subscriber station belongs or recognize the segment to which the subscriber station belongs in the cell. In order to recognize the cell or the segment in the cell to which the subscriber station belongs, the subscriber station analyzes a signal broadcasted from the wireless communication system.
Recently, a wireless communication system employing an orthogonal frequency division multiple access (OFDMA) scheme has been proposed in Institute of Electrical and Electronics Engineers (IEEE) 802. 16 standardization group (the IEEE 802.16 standard) in order to transmit data at a high speed. FIG. 1 is a block diagram illustrating a construction of a wireless communication system employing the OFDMA scheme. A subscriber station (not shown) having a mobility is located in a cell 10 or 20 and communicates with a backbone network 40. The backbone network 40 is connected to an authentication and service authorization server (ASA) 50 which provides authentication and service authorization to the subscriber station (SS).
A preamble signal broadcasted from an OFDMA system according to the IEEE 802. 16e standard, provides information for performing a cell search and initial synchronization. The initial synchronization is estimated from a repeated pattern contained in the preamble signal. In order to generate such a preamble signal having a repeated pattern in a multi-carrier system such as the OFDMA system, it is necessary to periodically insert a zero into a transmission signal of a frequency domain and to change the transmission signal into a time domain signal using an inverse fast Fourier transform (IFFT) before the transmission signal is transmitted.
FIG. 2 is a view showing an example of frequency use in a wireless communication system having a multi-sector environment. A preamble signal is obtained by inserting a signal into only one of three subcarriers and zero into the other two of the three subcarriers in a frequency domain and then performing IFFT for the three resultant subcarriers, so that the preamble signal includes three time repetition of a predetermined signal pattern in a time domain.
Meanwhile, as described above, a cell in the wireless communication system can be divided into multiple segments. The preamble signal includes a cell ID and 96 pseudo-noise (PN) sequences distinguished according to segments as information for a cell search. Each PN sequence includes 284 bits and is modulated to a frequency having a three interval in a frequency domain. From among the every three subcarriers, the position of the bit into which the signal is inserted is determined according to segments. From among 96 PN sequences, PN sequences of # 0 to 31 correspond to segment #0 (12), # 0 12 PN sequences of # 32 to 63 correspond to segment #1 (14), and PN sequences of # 64 to 95 correspond to segment #2 (16). As used above, 0, 1, and 2 refer to respective segment numbers and 12, 14, and 16 are reference numbers corresponding to their respective segment numbers.
The respective cells and segments are distinguished from each other by preamble signals made using a scheme as described above. Finding a cell to which the subscriber station belongs is identical to finding the preamble signal of the cell. The subscriber station converts the preamble signal into a signal of a frequency domain by the fast Fourier transform (FFT). The preamble signal of the frequency domain is subjected to a cross correlation calculation process with PN sequences. Herein, the PN sequence having the largest correlation value is determined as the PN sequence of a current cell.
FIG. 3 is a flowchart illustrating a conventional procedure for a cell search. When a subscriber station receives a preamble signal, the subscriber station obtains a correlation value by correlating known PN sequences with the preamble signal in step 102. In this case, since the subscriber station must know the segment to which the subscriber station belongs in the cell to which the subscriber station belongs, the subscriber station correlates the preamble signal with the whole PN sequences. Then, the subscriber station determines whether or not the number of times of correlation-value calculation is equal to or more than the number of the whole PN sequences in step 104. As a result, when the number of times of correlation-value calculation is less than the number of the whole PN sequences, the subscriber station proceeds to step 108 and increases the number of times of correlation-value calculation by one and then returns to step 102. The subscriber station correlates the preamble signal with the whole PN sequences in the above-mentioned way by turns. Thereafter, when the number of times of correlation-value calculation is equal to or greater than the number of the whole PN sequences, the subscriber station proceeds to step 106. In step 106, the subscriber station selects the PN sequence having the largest correlation value from among the whole correlation values, for example, from among 96 correlation values.
FIG. 4 is a block diagram illustrating the conventional cell search apparatus. The cell search apparatus shown in FIG. 4 illustrates that the amount of calculations required to obtain the largest correlation value, for the purpose of finding the segment to which the subscriber station belongs in the cell to which the subscriber station belongs. That is, the cell search apparatus correlates all of the known PN sequences with the preamble signal of the frequency domain.
The cell search apparatus includes a first block 210 for correlating 32 PN sequences corresponding to segment #0, a second block 220 for correlating 32 PN sequences corresponding to segment #1, and a third block 230 for correlating 32 PN sequences corresponding to segment #2. In addition, the cell search apparatus includes a largest PN sequence selection unit 240 for selecting the PN sequence having the largest correlation value from among correlation values provided from the blocks 210, 220 and 230. As described above, the conventional cell search apparatus must correlate all of the known PN sequences with a preamble signal of the frequency domain.
According to such a conventional cell search apparatus, it is necessary to correlate all PN sequences corresponding to segments of each cell with a preamble signal, so that it takes an excessive amount of time to perform a cell search.